The skids on which helicopters rest when they are on the ground are exposed to severe wear because helicopters generally do not land exactly vertically but rather with a certain amount of forward or sideways motion. This imposes a considerable frictional stress on the skids when the helicopter touches the ground. Skids are expensive components and have weight limitations; consequently, it has been customary to attach wear-resistant skid shoes to the underside front, center and rear, or even the entire length of the skid tubes. These shoes typically have a welded hard-faced surface, and they are readily replaceable when they wear through.
As shown in U.S. Pat. No. 4,544,116 to Shwayder, it has previously been proposed to use as helicopter shoes a steel channel containing crushed tungsten carbide particles in a copper-based binder. Similarly, in U.S. Pat. No. 3,117,845 to Reed, a steel plate is coated with a coating of tungsten carbide particles distributed in a copper-based binder and covered with a flame-sprayed copper-nickel alloy.
The prior art wear surfaces described above have several disadvantages. For one, the crushed tungsten carbide particles embedded in a softer binder form a rough surface which produces considerable friction. This is undesirable because a pilot has better control over the helicopter if it slides smoothly on the ground when landing. Secondly, both the channel of U.S. Pat. No. 4,544,116 and the plate of U.S. Pat. No. 3,117,845 have sharp side edges. These can catch in a gouge or groove in a runway surface when there is a transverse component to the helicopter's motion on landing, and cause a serious accident. Thirdly, as the surface wears, tungsten carbide particles break loose from the binder and reduce the life of the shoe. Fourthly, the clamp mounting of U.S. Pat. No. 4,544,116 is unsatisfactory because it allows the shoe to shift on the skid under strong impacts.
It has previously been proposed in the street sweeping industry to braze small blocks of tool-tip grade tungsten carbide into a groove milled in a sweeper drag shoe. This approach, however, is unsuitable for helicopter skids because it suffers from the same disadvantages as the channel of U.S. Pat. No. 4,554,116. Furthermore, tool-tip grade tungsten carbide tends to fracture under the impact of a helicopter landing.
There consequently exists a need for a smooth, wear-resistant helicopter skid shoe that has no sharp edges that can catch on lateral movement, and that can be bolted directly to the skids.